Nonvolatile memories are electronic devices of very great importance. They can be used in particular as mass memories, replacing the computer hard disks, but also to store the configuration of a programmable digital component such as a user-programmable gate array, or FPGA (field-programmable gate array), replacing the volatile memories (SRAM) currently used in the great majority of these components.
A nonvolatile memory ought to have a high storage density, extremely fast access times (equivalent to those of a conventional static read-only memory, SRAM), low consumption and a long information retention time. At present, a number of memory technologies are currently being studied and validated, at different stages of maturity. Among these technologies, those based on the effects of magnetic kind seem particularly promising. For a review of the current state of development of the magnetic nonvolatile memories, reference can be made to the article by Mark H. Kryder and Chang Soo Kim, “After Hard Drives—What Comes Next?”, IEEE Transactions on Magnetics, vol. 45, No. 10, pp. 3406-3413, October 2009.
The list of technologies drawn up by this publication omits a principle which is inciting the interest of many laboratories: magnetoelectric memories, in which the information, stored in a magnetic form, would be written by an electrical command of low energy, typically of voltage type, and read magnetically. In principle, such memories could exploit so-called multiferroic materials, having a ferroelectric phase and a ferromagnetic phase coupled together. The theory predicts a small number of these materials having these characteristics intrinsically, mainly because of constraining conditions of crystalline symmetry which it is necessary to have for the two “ferroelectric” and “ferromagnetic” effects to exist simultaneously. No material fulfilling the right conditions and having sufficiently marked effects at ambient temperature currently exists.
Another solution is the combination of magnetic and ferroelectric materials, or more generally piezoelectric or electrostrictive materials, coupled through mechanical stresses so as to simulate a multiferroic behavior.
The article by V. Novosad et al., “Novel magnetostrictive memory device”, J. Appl. Phys., Vol. 87, No. 9, 1 May 2000, and the U.S. Pat. No. 6,339,543, describes a magnetoelectric memory using as information storage elements magnetic particles of ellipsoid form, of sufficiently small size to consist only of a single magnetic domain. Because of their anisotropy of form, these particles exhibit two stable magnetization orientations, mutually opposed and aligned with the great axis of the ellipse. Lines of electrostrictive material crossing at 90° are deposited on a substrate, a magnetic particle being positioned at each intersection with its great axis oriented at 45° relative to the lines. By applying to the electrostrictive lines signals of suitable voltage, it is possible to induce a rotating mechanical stress at a determined particle; by reverse magnetoelastic effect, this in turn induces a rotation of the magnetization. If their timing and their amplitude are chosen shrewdly, these voltage signals can provoke a switchover of the magnetization of this particle from one stable state to the other.
Such a device has a number of drawbacks. Firstly, it is very sensitive to the synchronization of the electrical signals, which complicates its control and limits its response time; this is due to the fact that the field of mechanical stresses has to rotate to “guide” the rotation of the magnetization vector. Secondly, a write operation can only overturn the state of magnetization of a particle; this means that, to write a “0” or a “1” in a memory cell, it is necessary to first read its content in order to determine whether such an overturning should or should not be performed.
The article by M. Overby et al., “GaMnAs-based hybrid multiferroic memory device”, Applied Physics Letters 92, 192501 (2008) describes another magnetoelectric memory based on an epitaxial layer of GaMnAs (magnetic semiconductor) deposited on a substrate of GaAs which is thinned and fixed to a piezoelectric crystal. The layer of GaMnAs exhibits two easy magnetization axes, along respective crystalline directions, which correspond to two stable magnetization directions. The application of a mechanical stress, made possible by the piezoelectric crystal, makes it possible to switch over from one stable state to the other. Such a device is complex to fabricate, because the knowledge of the two easy magnetization axes presupposes a fine control of the epitaxial growth conditions. Moreover, despite the application of a mechanical stress, an energy barrier always remains between the two stable states: this means that the switchover has to be thermally assisted, or is performed by tunnel effect, which is likely to limit the speed of response of the memory. The operation of the device has been demonstrated only at cryogenic temperatures.